Schools are buying mobile detectors - nifty devices that pick up the presence of cell phones, bugs and hidden cameras, and are more commonly associated with espionage than tutelage.

Global Gadget UK usually sells its pound;149.99 mobile phone detectors to prisons, where they are used to stop inmates sending text messages to buy drugs. But company director Michael Menage started marketing them to schools three weeks ago. He hit on the idea after a teacher friend complained about mobile phones in the classroom.

He said: “My teacher friends all say it’s really tricky to deal with. How do you know if one of the kids has their phone on?

“I thought it would be useful when they’re taking their exams.”

The appliance cannot pick up the presence of a mobile when it is not in use. But it can alert staff if pupils are sending text messages under the table or making calls behind their exercise books. The detector vibrates, bleeps or flashes a red warning light whenever a phone is used.

A talking detector even allows teachers to record their own “turn that phone off now” message.

Teachers invigilating A-level and GCSE exams at All Hallows high school in Preston have been taking advantage of the new technology. Michael Flynn, headteacher, says the detector is a brilliant deterrent.

“Sometimes children simply forget they have their phone on them,” he said.

“At first we didn’t tell the pupils we were using a detector. They thought the staff had suddenly developed psychic powers.

“But they know now, and they have really got the message.”

At Heathland school in Hounslow teachers are still getting to grips with their gadget, trying it out on each other in the staffroom.

“We’ve worked out how to put the batteries in,” said Nigel Roper, senior deputy head. “But it’s early days yet.”